Gamma oryzanol (oryzanol) is a natural antioxidant that is a mixture of steryl ferulates, which is found in rice, corn, and barley oils. Oryzanol contributes to the stability of oil in storage and food applications. Specifically, the oryzanol in rice bran oil (RBO) functions as a natural substitute for synthetic antioxidant additives normally used in vegetable oils to enhance or prolong shelf life. Oryzanol, therefore, is a useful substitute for synthetic antioxidant additives such as propyl gallate, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and tertiary butylated hydroquinone (TBHQ). In addition, evidence suggests that oryzanol may provide numerous health benefits including reducing serum LDL cholesterol.
In crude rice bran oil (CRBO), oryzanol is present in amounts of 1 to 3% of the CRBO; however, conventional caustic refining methods strip away a substantial amount of oryzanol from the final product due to over-treatment of the crude oil with alkali solution(s) during FFA neutralization. Thus, many of the benefits of oryzanol are not retained in RBO refined using conventional alkali treatment.
Crude edible vegetable oil, which includes neutral oil, non-triglyceride materials, such as free fatty acids (FFA), hydratable and non-hydratable phospholipids, moisture, waxes, peroxides and related products, color pigments, and dirt, goes through various processing steps before becoming refined oil product.
During conventional edible vegetable oil processing, a series of steps, often collectively referred to as “refining” are commonly included as part of overall vegetable oil processing. These steps can include one or more of the following: (a) degumming, which is the removal of phosphatides, (b) removal of FFA via neutralization, (c) bleaching, which is the removal of colorant pigments, residual soaps and gums, and pro-oxidant metals, (d) dewaxing, which can occur at different stages throughout the refining process, (e) winterization, which is also known as stearine removal, and (f) deodorization, which is the removal of undesirable volatile impurities, odors, and flavors from the oil. In lieu of degumming, the crude oil can be pretreated based on the non-hydratable phosphatide content of the crude oil with an amount of mineral, organic, or combinations of acids such as phosphoric or citric acids. Typically the amount of mineral or organic acid for the pretreatment step ranges from 300-1000 ppm.
In some cases, depending on plant design or producer's preference, two or more of the above steps may be combined.
In the deodorization step, odorous and volatile impurities as well as residual FFA, small amounts of triglycerides, and other organic impurities will be carried off or stripped out via steam distillation under vacuum as distillate. The temperature and pressure used for the deodorization step will define the amount of distillate produced at this stage.
For the neutralization step, the acidified CRBO can be treated directly with a predetermined caustic dose, or first degummed to remove hydratable and non-hydratable phospholipids from the crude oil and then treated with a caustic dose.
In either case, the conventional caustic treatment applied during the neutralization step includes a caustic dose that contains an amount of caustic that is in excess of the theoretical amount required to neutralize all FFA.
Determination of the excess caustic treatment quantity per conventional alkali refining is based on (a) the concentration of FFA in the crude oil, (b) the amount of acid addition for preconditioning of the crude oil, and (c) the excess caustic over theoretical amount for different types of oils as shown in the formula below. For example, the strength of caustic (NaOH) solution is determined by solution's specific gravity, which is expressed in degrees Baume (Be°).
      %    ⁢                  ⁢    Caustic    ⁢                  ⁢    Treat    =                                                                        (                                  %                  ⁢                                                                          ⁢                  FFA                  ×                  0.142                                )                            +                              %                ⁢                                                                  ⁢                Excess                ⁢                                                                  ⁢                Caustic                            +                                                                                          Amount                ⁢                                                                  ⁢                Pretreatment                ⁢                                                                  ⁢                Acid                            ⁢                                                                                                                      Addition              ⁢                                                          ⁢              to              ⁢                                                          ⁢              Crude                                                  %        ⁢                                  ⁢        NaOH        ⁢                                  ⁢        in        ⁢                                  ⁢        Caustic              ×    100  
The concentration of the FFA in the crude oil is measured by a standard titration method known to those of ordinary skill in the art
The theoretical amount of caustic is calculated from the ratio of the molecular weight (MW) of the caustic material, such as NaOH to the MW of oleic fatty acid.
      Molecular    ⁢                  ⁢    Weight    ⁢                  ⁢    Factor    =                    NaOH        ⁢                                  ⁢        Molecular        ⁢                                  ⁢        Weight                                                                        Oleic                ⁢                                                                  ⁢                Fatty                ⁢                                                                  ⁢                Acid                                                                                                                                          ⁢                                  Molecular                  ⁢                                                                          ⁢                  Weight                                                                    ⁢                                        =                  40        282            =      0.142      
The amount of excess caustic used in the conventional methods varies depending on the type of oil and past refining experience with the particular oil(s); however, the total amount of caustic used is always greater than the theoretical amount needed to neutralize the FFA plus the amount of added free acid. These amounts are well known to those of ordinary skill in the art and can be found in standard refining reference books. See Robert R. Allen et al., Bailey's Industrial Oil and Fat Products, Fourth Edition, 1982. Daniel Swern ed., John Wiley & Sons.
U.S. Pat. No. 6,197,357 and Mattikow teach production of refined oils rich in at least one unsaponifiable component by refining the CRBO via a weak acid salt. See M. Mattikow, Development in the Refining of Oils with Sodium Carbonate, JAOCS 25 (6) pp. 200-203 (1948). These methods reportedly result in the retention of about 75 to 100% of at least one unsaponifiable component in the refined oil. However, U.S. Pat. No. 6,197,357 and Mattikow teach an excess treatment of a weak acid salt in order to effectively neutralize the FFA present in the CRBO.
Japanese Patent Application 10-293157 (JP 10-293157) teaches using a combination of a weak alkali & buffer solution instead of strong alkali solutions to produce, according to JP 10-293157, RRBO with 80% or more of the oryzanol originally present in the CRBO. This application also teaches an excess treatment with weak alkali and buffer solution in order to effectively neutralize the FFA present in the CRBO.
As previously noted, one of the major problems with refining RBO by typical conventional alkali or caustic refining methods is the loss of 93% or more of the oryzanol in the original crude oil. Table 1 below lists the cumulative loss percentage of oryzanol content at various conventional processing steps as discussed in the Effect of Refining of Crude Rice Bran Oil on the Retention of Oryzanol in the Refined Oil. A. G. Gopala Krishna, Sakina Khatoona, P. M. Shiela, C. V. Sarmandala, T. N. Indirab, and Arvind Mishrac. Effect of Refining of Crude Rice Bran Oil on the Retention of Oryzanol in the Refined Oil, JAOCS, Vol. 78, No. 2 (February 2001).
TABLE 1Oryzanol Content afterProcessProcessing Step (%)Loss (%)Control Rice Bran Oil1.86—(Free Fatty Acid, 6.8%)Degumming1.841.1Dewaxing1.755.9Control Rice Bran Oil +0.1094.6alkali treatmentDegummed Rice Bran Oil +0.1194.1alkali treatmentDewaxed Rice Bran Oil +0.1393.0alkali treatmentAs such, there is a need for an economical refining method that results in substantial retention of oryzanol in the refined oil.